For an electric power converter of a equipment represented by a hybrid electric vehicle, a converter, for example, is known which has a configuration given in FIG. 25, a block diagram sowing a circuit configuration of a related power converter. More specifically, the electric converter has a configuration including a DC power supply unit 102, a DC to DC converting circuit 104, a DC circuit 105 and a DC to AC converting circuit 106. The DC power supply unit 102 is formed with an AC to DC converting circuit converting three-phase AC electric power generated by an AC generator 101 to DC electric power. The DC to DC converting circuit 104 raises the DC voltage of a battery 103 connected between a positive electrode side line Lp and a negative electrode side line Ln of the DC power supply unit 102. The DC circuit 105 has a DC link capacitor connected in parallel to the DC to DC converting circuit 104. The DC to AC converting circuit 106 converts DC power outputted from the DC circuit 105 to AC power. The three-phase AC power outputted from the output points of the DC to AC converting circuit 106 is supplied to an AC motor 107.
In such a hybrid electric vehicle, though not illustrated, wheels of the vehicle are driven to rotate by the AC motor 107 while being connected to the AC motor 107 directly or through a reduction gear. Moreover, when an electric vehicle is in braking or in running a downhill, the AC motor 107 is operated as a generator to bring a power flow into a state of regeneration in which the power flows in the direction from the side of the AC motor 107 to the side of the battery 103 or the AC generator 101. This makes the battery 103 carry out output of complementary power (battery discharging) for driving the AC motor 107 and recovery of regenerated energy (battery charging) under a regenerating condition in which power is supplied from the AC motor 107. A this time, with a reactor forming the DC to DC converting circuit 104 and semiconductor switching devices and diodes connected in inverse parallel to them, the battery voltage is raised to the voltage of the DC circuit 105 at discharging of the battery and reduce the voltage of the DC circuit 105 to the battery voltage at charging of the battery.
With a similar operation, an exchange of power becomes possible also between the battery 103 and the AC generator 101. This, with the circuit configuration provided as given in FIG. 25, enables mutual exchanges of power among the AC generator 101, the AC motor 107 and the battery 103.
Of the electric power converters with their configurations similar to that shown in FIG. 25, an electric power converter is also known which has a configuration having an electric power converting unit of three-level circuit (see JP-A-5-308778, for example). In the electric power converter, a DC circuit is formed with a pair of capacitors connected in series between the positive electrode side line Lp and the negative electrode side line Ln, and the electric power converting unit is provided to have a configuration in which three sets of inverter arms, each formed with four switching devices connected in series, are connected in parallel. From the connection point of a second and third switching devices in each of the inverter arms, a connection line to an AC output terminal is derived. The connection point of a first and second switching devices is connected to an intermediate potential point as a connection point of the capacitors in the DC circuit through a diode. The connection point of the third and fourth switching devices is also connected to the intermediate potential point in the DC circuit through a diode.
Moreover, an electric power converter is also proposed in which a voltage of a battery for electricity accumulation or a capacitor is used for a voltage of one phase of three phases of an output with its value taken as approximately one-half of that of the voltage of a DC power supply and, along with this, an inverter controlling unit is provided which carries out control so that the voltages of the rest two phases of the three phases of the output are outputted with the electric potential of the battery for electricity accumulation or the capacitor taken as a reference (see JP-A-2005-57938, for example). In the electric power converter, the voltage of the battery for electricity accumulation or the capacitor is utilized as it is as a voltage of a terminal of a motor to omit semiconductor switching devices for one phase so that a three-phase motor is driven with four semiconductor switching devices.
However, in the example of the related electric power converter shown in FIG. 25, the DC to AC converting circuit 106, having a two level converter circuit configuration, causes voltage variation to become larger with each on-off control of the switching device. This increases a switching loss of the switching device (an IGBT and a diode) and, along with this, also on the side of the AC motor 107, increases a ripple in a flowing current to increase a harmonic loss due to carrier frequency components, which results in a decrease in efficiency of the AC motor to be an unsolved problem. For solving the unsolved problem, there is an idea of providing the configuration of the DC to AC converting circuit 106 with a three-level circuit as in the example of the related converter described in JP-A-5-308778. However, the DC to AC converting circuit 106 with a configuration normally provided with the three-level circuit causes controls of the twelve switching devices forming the DC to AC converting circuit 106 to be complicated to increase the load on the control circuit controlling the DC to AC converting circuit 106, which becomes a new problem.
Moreover, in the example of the related electric power converter shown in FIG. 25 and the examples of the related electric power converter described in JP-A-5-308778 and JP-A-2005-57938, an electric vehicle normally has a battery mounted in which tens of units of batteries each having an output voltage of several volts are connected in series to be provided as a battery with an output voltage of hundreds of volts. Thus, when only any one unit causes failure for some reason, the battery units connected in series causes the whole of the battery system to become unusable. Therefore, in a battery system with battery units simply connected in series as shown in FIG. 25, there is also an unsolved problem in that the reliability of the system can not be improved.
Furthermore, in the example of the related electric power converter shown in FIG. 25 and the examples of the related electric power converter described in JP-A-5-308778 and JP-A-2005-57938, when the capacitor CO in the DC circuit 105 causes failure, smoothing of the DC voltage becomes impossible and a storage component of energy is lost. Thus, there is also an unsolved problem in that this disables the converter from being normally operated as an electric power converter.
Still further, in the example of the related electric power converter shown in FIG. 25 and the examples of the related electric power converters described in JP-A-5-308778 and JP-A-2005-57938, when an IGBT or a diode in any phase forming the DC to AC converting circuit 106 becomes abnormal, a three-phase balanced operation of the AC motor 107 becomes impossible. Thus, there is also an unsolved problem in that this makes the AC motor 107 incapable of being normally operated.
Accordingly, the invention was made by giving attention to the unsolved problems in the above examples of the related electric power converters with an object of providing an electric power converter which can ensure a normal operation of an AC motor as required even in the case when a battery, a capacitor in a DC circuit or switching devices in an electric power converting unit become abnormal while making the electric power converter and the AC motor highly efficient.